Displacement sensors, such as microphones and pressure sensors, are well-known. Displacement sensors based on capacitive, impedance, and optical measurements have been developed. Optical displacement sensors are particularly attractive as they overcome many of the limitations of capacitive and impedance measurement techniques, such as low sensitivity, the need for high-voltage biasing, poor electrical isolation, or response nonlinearities.
Many optical-displacement sensors known in the prior art operate by detecting light reflected by an optical element that changes its reflectivity in response to an environmental stimulus, such as pressure differential, sound, vibration, etc. The detected light is converted into an electrical signal. This signal is a function of the reflectivity of the optical element, and, therefore, a function of the stimulus as well.
A Fabry-Perot interferometer has served as such an optical element. The reflectivity of a Fabry-Perot interferometer depends on the spacing between its two, substantially-parallel, partially-reflective surfaces and its operating wavelength, λ, (i.e., the wavelength, λ, of the light on which the interferometer operates).
The spacing between the partially-reflective surfaces is referred to as the “cavity length” of the Fabry-Perot interferometer. In order to form a Fabry-Perot interferometer that is sensitive to sound, etc., one surface of the Fabry-Perot interferometer is a surface of a movable membrane. When the movable membrane moves in response to incident sound, the cavity length changes and, therefore, so does the reflectivity of the Fabry-Perot interferometer. As a result, the electrical signal based on the detected light is a function of the acoustic energy incident on the Fabry-Perot interferometer.
The relationship between its initial cavity length (i.e., its cavity length in the absence of environmental stimulus), L0, and the operating wavelength has significant impact on the interferometer's performance in many applications. Therefore, prior art Fabry-Perot interferometers are typically designed to have a specific value of L0 (i.e., a “design value”). In practice, fabrication and packaging variations lead to a deviation between the actual and the design value of L0. This deviation generally degrades interferometer sensitivity and signal-to-noise ratio (SNR). It has been necessary in the prior art, therefore, to provide a means for individually tuning the cavity length of each Fabry-Perot interferometer to achieve the proper relationship to wavelength in order to overcome this variation.
In prior-art microphones, L0 is tuned electrostatically. A voltage applied to electrodes that are added to both the deformable membrane and the substrate attracts the deformable membrane toward the substrate, thereby reducing L0. The use of an electrostatic actuator to tune L0 has three main drawbacks: 1) deforming the membrane increases its internal stress, thereby increasing its resonant frequency and affecting interferometer response; 2) a high electric-field is required to generate sufficient force to deflect the membrane; and 3) the addition of an electrostatic actuator complicates the fabrication of the Fabry-Perot interferometer, which increases the cost and complexity of the microphone.
In order to induce an electric field of sufficient strength to tune the membrane, the Fabry-Perot interferometer must either have a short cavity length or a very high voltage (typically about 200-300 V) must be used. The use of a high voltage is undesirable in most applications, since it requires high-power electronics and/or charge pumps. High-power electronics are known to have poor reliability as compared to that of low-power electronics. Also, the quiescent power dissipation of high-power electronics leads to significant power consumption; this is particularly undesirable in applications such as hearing aids and cell phones. Finally, the use of a high-voltage actuator creates a potential safety hazard, which is particularly undesirable in the case of hearing aid microphones.
To avoid high-voltages, prior-art optical microphones have instead relied on a short cavity length to attain sufficient electric field strength. A short cavity length, however, results in undesirable dynamic effects that negatively impact Fabry-Perot interferometer performance, such as the well-known “squeeze-film” effect. The squeeze-film effect arises from the motion of one of two closely-spaced surfaces. It is caused by the compression and decompression of a trapped volume of gas, which is present between the surfaces. This results in generation of “noise pressure,” which contributes directly to the noise in the output signal, thereby degrading SNR.
A displacement sensor having high dynamic range and high sensitivity but does not require a Fabry-Perot interferometer whose initial cavity length is mechanically-tunable would, therefore, be a significant advance in the art.